Quaternary Science Journal - 230Th/U-dating of intergarcial and interstadial fen peat and ligni... by Geozon Science Media

Eiszeitalter und Gegenwart Quaternary Science Journal

57/1–2

77–94

Hannover 2008

230

Th/U dating of interglacial and interstadial fen peat and lignite: Potential and limits )

MEBUS A. GEYH *

Abstract: The state-of-the-art and potential of the 230Th/U disequilibrium method are discussed for the dating of fen peat and lignite. Recommendations are given for the collection of suitable samples. The numerous interfering factors in 230Th/U dating of fen peat show that a rigorous examination of the reliability of the measured data sets is required if reliable 230Th/U ages are to be obtained. The accuracy of such 230Th/U ages allows a reliable correlation of interglacial and interstadial deposits to the warm periods documented in the SPECMAP timescale but does not yet allow detailed temporal resolution. [230Th/U-Altersbestimmung interglazialer und interstadialer Niedermoortorfe und Ligniten: Potential und Grenzen] Kurzfassung: Der Stand und die Zukunft der Altersbestimmung von Niedermoortorf und Lignit mit der 230 Th/U-Ungleichgewichtsmethode werden diskutiert. Empfehlungen zur Auswahl der Probeentnahmestellen und zur Entnahme von geeigneten Proben werden gegeben. Aufgrund der zahlreichen negativen Einflussmöglichkeiten ist eine gründliche Eignungsprüfung der gemessenen Aktivitätsverhältnisse von Uran und Thorium solcher Proben notwendig, um zuverlässige Alter bestimmen zu können. Die Datierungsgenauigkeit ist ausreichend, um interglaziale und interstadiale Ablagerungen sicher den Warmperioden der SPECMAPZeitskala zuzuordnen, noch nicht aber, um sie zeitlich auflösen zu können. Keywords: 230Th/U dating, fen peat, lignite, minimum age

1 Basic principles of 230Th/U dating Peat deposits are important archives of Pleistocene climate change but correlation of individual records with long-term archives such as marine oxygen isotope records (SPECMAP; MARTINSON et al. 1987) remains tentative. Correlations based on palynological and stratigraphic studies are usually based on certain pre-assumptions and can hence not provide any independent age constrains. Sound numerical dating methods seem to be suitable tools to achieve a reliable correlation between the marine and terrestrial climatic records. One of them is the 230Th/U dating

method. It is suitable for all materials that accumulated uranium during their formation and have not been affected by post-sedimentary processes that would mobilise uranium and thorium. When this is the case, the sampled material can be expected to have behaved as a closed system with respect to uranium and/or thorium during ageing. Moreover, it should not be older than 350-500 ka (IVANOVICH & HARMON 1992; BOURDON et al. 2000). The 230Th/U dating method is based on the radioactive decay series of 238U. The first longlived daughter isotope 234U (τ = 2.4525×102 ka, CHENG et al. 2000) decays into the isotope 230 Th (τ = 75.69 ka, CHENG et al. 2000) with the

*Address of autor: M. A. Geyh, Rübeland 12, 29308 Winsen/Aller, Germany. E-Mail: Mebus.geyh@t-online.de

0,0 0

200.000

400.000 600.000 MEBUS A. GEYH

800.000

1.000.000

230

Th/

238

U and

234

238

U AR

1,4 1,2 1,0

234

radioactive equilibrium

U/238U AR

0,8 0,6 0,4

230

Th/238U AR

0,2 0,0 10.000

100.000

230

1.000.000 Th/U age (a)

Fig. 1: Change in the 230Th/238U and 234U/238U activity ratios with increasing age for an initial 234U/238U AR of 1.15 (corresponding to the mean of marine sediments). Abb. 1: Änderung der 230Th/238U- und 234U/238U-Aktivitätsverhältnisse mit der Alterung für ein initiales 234U/ 238 U-Aktivitätsverhältnis von 1,15 (entsprechend dem Mittelwert mariner Sedimente).

emission of an alpha particle. The half-life of ratio (AR) = 0. The radioactive disequilibrium 238 U (τ = 4.468×109 a) is considerably longer between 230Th and 238U continues and evolves than that of 234U. As a consequence, radioac- during ageing. A new radioactive equilibrium tive equilibrium is established after about one is approached after 350-500 ka. This process million years in most geologically old rocks can be used toused datetosample material andand is deprocess can be date sample material is described by Eq and sediments. At radioactive equilibrium, all scribed by Equation 1, which has to be solved solved iteratively. Activity ratios are given in square brackets and � members of the decay series have the same spe- iteratively. Activity ratios are given in square cific activity (which is the decay rate massto date brackets λ = ln process canper be used sampleand material and2/τ: is described by Equation 1, which has to be unit), i.e., the activity ratiossolved of anyiteratively. two of the 230 � 234and � given Activity ratios are Th � in square U � =� ln 2/�: �230 � � brackets �t (1) ) � �� 238 � 1�� (1 � e � ( � series members are equal to one. � 238 � � (1 � e U U � � � � � � � 230 234 Radioactive equilibrium can be disturbed by 230 234 geochemical processes if the members � U � Thof� the � �230 � (1 � e � � �t ) � �� 238 � 1��230 ) � � (1 � e � ( � � � )�t ) � 238 �proradioactive series have certain(1geochemical 238 The change U234AR with age t is shown in Figure 1 as � Uin the� �Th/ � U � 230 � � perties. An example is the uranium and thorium 234 238 U ARinof the 1.15,230which fort most isotopes of the uranium-238 decay series. U(VI) TheU/change Th/238isUcharacteristic AR with age is marine carbo 234a wide range betwe 230 238 of samples from terrestrial environments covers ions are soluble in oxygenated while tho-Th/shown assuming an 1initial Thewater change in the U AR in withFigure age t is1shown in Figure assuming U/ an initial 238 230 238 is characteristic for 234 rium is practically insoluble. Hence, U AR of 1.15, which 234 238 during wea238 Th/ U AR takes place between 50,000 and rapid change in the U/ U AR of 1.15, which is characteristic for most marine carbonates. The U/ U AR thering, uranium tends to be dissolved in water most marine carbonates. The 234U/238U AR of maximum age that canabewide measured by any dating the mi of samples from terrestrial environments covers range between 1 andamethod 20. Theismost and thorium bound by clay minerals. samples from terrestrial environments covers 230 230 238 Th/U dating methods ForUthe radiometric spectrometric Uranium dissolved in groundwater range between 1 mass and 50,000 20. The Th/wide AR takes placeand between andmost aboutrapid 200,000 years. The rapid changecan in thebe 230 238 incorporated into new systems. Examples are change in the Th/ U AR takes place between about 350 ka and about 500 ka, respectively (Chapter 7). maximum age that can be measured by any dating method is the minimum age of the sample speleothems, foraminifera and fen peat. The 50,000 and&about 200,000 years. The maximum 230 KAUFMAN BROECKER (1965) introduced an isotope-ratio evolutio Th/U dating methods, this maximum age is For theassumes radiometric spectrometric most simple 230Th/U dating model thatand mass age that can be measured by any dating method 234 238 230 238 U/ U AR is plotted the Th/ U AR. It shows the varia aboutonly 350 ka and about 500 ka,minimum respectively (Chapter the new system initially contains uranium. is the age of versus the 7). sample. For the radi234 230238 238 U/ Th/U U AR yields a separate with increasing age. Each initial evolution Radioactive disequilibrium exists between U ometric and mass dating KAUFMAN & BROECKER (1965) introduced anspectrometric isotope-ratio plot in which the evo 230 and 230Th during ageing. The234 numerical Th/U methods, this maximum age is abouti.e., 350atka and age. The ARs 238 230 238 point (1,1) at radioactive equilibrium, infinite U/ U AR is plotted versus the Th/ U AR. It shows the variation of these two ARs clock starts at zero and a 230Th/238U activity about 500 ka, respectively (Chapter 7). 230

230

234

lines corresponding ages, which line. are termed "isochr U/238ofUtheir AR yields a separate evolution All meet the with increasing age. Eachstraight initial 234 point (1,1) at radioactive equilibrium, i.e., at infinite age. The ARs of coeval samples fit 230 Th/U dating (Fig. 2). 2 Fen peatages, and which ligniteare fortermed straight lines of their corresponding "isochrons"

Th/U dating of interglacial and interstadial fen peat and lignite:Potential and limits

230

KAUFMAN & BROECKER (1965) introduced an isotope-ratio evolution plot in which the 234U/ 238 U AR is plotted versus the 230Th/238U AR. It shows the variation of these two ARs with increasing age. Each initial 234U/238U AR yields a separate evolution line. All meet the point (1,1) at radioactive equilibrium, i.e., at infinite age. The ARs of coeval samples fit straight lines of their corresponding ages, which are termed “isochrons” (Fig. 2).

contain have an extremely large capacity for forming complexes with uranium dissolved in groundwater. Moreover, in the reducing milieu of peat, soluble U(VI) is reduced to U(IV) and bound in very stable, immobile uranyl organic complexes. As a result fen peat may have uranium concentrations of up to 100 ppm. The following scenario of fen peat formation is the basis for 230Th/U dating of fen peat and lignite. Figure 3 shows a schematic section of a growing fen moss in a shallow lake. During its growth, uranium dissolved in groundwater enters the moss and is tightly bound to fulvic and humic acids. Before the peat has been compacted, groundwater can enter even deeper parts of the moss. In the terminal stage, organic material fills the entire volume of the lake. In the radioactive glacial period, the organic deposits succeeding areequilibrium covered by mineral soil and compacted. Figure 4 shows a section of interglacial fen peat sandwiched between detrital material above and below. The uranium dissolved in the groundwater seeping through the bottom and top mineral cover layers during the period of ageing is continuously absorbed in the bottom and top parts of the peat. It has been empirically found that the corresponding rim layers are 1,000,000 230 seldom thicker than 10 cm. The organic materiTh/U age (a) al in the rim layers behaves as an open system with respect to uranium and is not datable. As a

U/ 238 U AR

The history of 230Th/U dating of fen peat began with a study by TITAYEVA (1966). Her efforts failed as the analyzed Holocene peat was too 1.4 young for 230Th/U dating. But TITAYEVA recognised that “excess” (detrital) thorium and ura1.2 234 238 U AR into account for 230Th/U nium have to U/ be taken 1.0 dating of fen peat. The first 230Th/U ages of this 0.8 material were published by VOGEL & KRONFELD (1980). Because the ages were not corrected for 0.6 Th/ 238U ARas suggested by detrital thorium and230uranium, 0.4 TITAYEVA (1966), their results were not acceptable by Quaternary geologists. The first chro0.2 nostratigraphically satisfactory 230Th/U ages of 0.0 fen peat were determined by VAN DER WIJK et al. 10,000 100,000 (1986) and HEINJES & VAN DER PLICHT (1992). Fen peat and lignite have a potential for 230Th/U 1 dating because the fulvic andFig. humic acids they

234

230

Th/238U and 234U/238U AR

2 Fen peat and lignite for 230Th/U dating

1,8 isochrons

1.8

1,7

100 ka

1,6 1,5

1.6 200 ka

1,4 1.4

1,3

300 ka

1,2 1.2

1,1 1,0

isochron of infinite age

evolution lines

0,9 0,0

0,2

0,4

0,6

0,8

1,0

1,2 230

Th/

Fig. 2

1,4 238

U AR

Fig. 2: Variation of the 234U/238U AR and the 230Th/238U AR with increasing age. Every initial 234U/238U AR yields an evolution line. All of them meet at the point (circle at 1,1) at infinite age, i.e. at radioactive equilibrium. The ARs of coeval samples plot on straight lines called isochrons (KAUFMAN & BROECKER 1965). Abb. 2: Entwicklung der 234U/238U- und 230 Th/238U-Aktivitätsverhältnisse mit wachsenden Alter. Jedes 234U/ 238U AR liefert eine eigene Entwicklungslinie, die wie alle anderen bei unendlichem Alter im Punkt 1,1 (Kreis) enden. Die Aktivitätsverhältnisse gleich alter Proben liegen auf Geraden, den Isochronen (KAUFMAN & BROECKER 1965).

MEBUS A. GEYH

� � � � � � � � ��

�� Fig. 3

Fig. 3: Accumulation of uranium bound to fulvic and humic acids in fen peat during the peat formation in a shallow lake. Abb. 3: Akkumulation von im Grundwasser gelöstem Uran an Fulvo- und Huminsäuren des Niedermoortorfs während der Entwicklung eines Niedermoores im flachen See.

consequence, groundwater entering the central part of fen peat more than 20-cm-thick should be free of uranium and the central part remains a closed system with respect to uranium. It is suitable for 230Th/U dating. The uranium and thorium data determined in a chronological study of the Netiesos interglacial site in Lithuania by GAIGALAS et al. (2005) is used in the following as a case study to illustrate the 230Th/U dating of fen peat and lignite. The authors determined an ESR age on freshwater mollusc shells of 101.5 ± 11.5 ka. Six samples were collected between 30 and 55 cm depth. All were analyzed using the L/L (leachate/ leachate) method (SCHWARCZ & LATHAM 1989; KAUFMAN 1993); the two outer samples (from 30 and 55 cm depth) were not analyzed with

the total sample dissolution (TSD) method (BISCHOFF & FITZPATRICK 1991; LUO & KU 1991). The U and Th isotope ratios were measured radiometrically at St. Petersburg University and summarized with 2σ standard deviations in a research report (Table 1). The standard deviations of these results deviate slightly from those published by GAIGALAS et al. (2005). The reason is unknown. 3 Factors that interfere with 230Th/U dating of fen peat and lignite There are two main factors which render 230Th/U age determination difficult or even impossible: 1. Open-system conditions with respect to uranium and thorium, as well as

Tab. 1: Isotopenergebnisse der Niedermoorproben von Netiesos in Litauen (GAIGALAS et al. 2005). No.

name

depth (cm)

ash (%)

5133 5134 5135 5136 5137 5138 5129 5130 5131 5132

L-1 L-2 L-3 L-4 L-5 L-6 T-2 T-3 T-4 T-5

30-35 35-40 40-45 45-50 50-55 55-60 35-40 40-45 45-50 50-55

50.4 51.9 49.7 49.3 47.3 45.8 51.9 49.7 49.3 47.3

238

U ± 2σ

(dpm/g) 0.537 ± 0.011 0.572 ± 0.018 0.494 ± 0.017 0.728 ± 0.022 0.640 ± 0.018 0.477 ± 0.008 1.582 ± 0.049 1.446 ± 0.034 2.009 ± 0.052 1.786 ± 0.039

Th/238U ± 2σ

U/238U ± 2σ

230

234

0.655 ± 0.032 0.617 ± 0.022 0.666 ± 0.029 0.657 ± 0.030 0.672 ± 0.031 0.755 ± 0.021 0.825 ± 0.032 0.900 ± 0.025 0.833 ± 0.025 0.843 ± 0.026

1.194 ± 0.068 1.079 ± 0.047 1.119 ± 0.053 1.088 ± 0.046 1.136 ± 0.044 1.105 ± 0.026 1.001 ± 0.044 1.014 ± 0.033 1.030 ± 0.038 1.029 ± 0.032

230

Th/232Th ± 2σ

1.824 ± 0.081 1.555 ± 0.043 1.437 ± 0.059 1.790 ± 0.100 1.748 ± 0.107 1.545 ± 0.053 1.584 ± 0.075 1.240 ± 0.026 1.448 ± 0.036 1.352 ± 0.043

Th/U dating of interglacial and interstadial fen peat and lignite:Potential and limits

230

2. Detrital contamination with allochthonous 230 Th.

� � � � � � � � ��

� � � � ��

� � � � � ��

� � ��

� ��

� � � �

� � � � ��

� � ��

� � � � ��

� � � � � � � � ��

Fig. 4 238

U (dph/g) &

234

238

U AR

Fig. 4: Schematic of a section through a46 layerash ofcontent50% 0,6 0,7 0,8 0,9 1,0 1,1 1,2 1,3 1,4 44 48 25 interglacial fen peat: Samples suitable for 230Th/U Netiesos L/L dating can only be expected in the central partNetiesos of L/L 30 the organic deposit. The top and bottom 10 cm in 35 most cases behaved as open systems with respect to 40 uranium during ageing and are, therefore, unsuitable for 230Th/U dating. 45

0,5

depth (cm)

Open system conditions: Fen peat and lignite theoretically offers a chemically ideal milieu for closed-system conditions with respect to uranium. In contrast to carbonates (e.g., corals, foraminifera, speleothem), the reducing milieu of the peat means that any uranium present is reduced to insoluble U(IV), which form stable complexes with inorganic and organic ligands, such as humic and fulvic acids. Moreover, movement of humic acids over large distances and, therefore, post-depositional displacement of uranium can be largely excluded in the confined conditions of interglacial peat deposits. Likewise, α-recoil only moves the uranium daughter isotopes over distances that are smaller than the common sample volume of several cm3 (HENDERSON & 0,4 SLOWEY 2000). According to the above-described 25 scenario for the formation of interglacial fen peat 30 deposits, the central part may be considered as 35 a closed system with respect to uranium and thorium. However, there are several possible 40si45 tuations in which this may not be true. Some fen peat deposits contain thin sand layers which may 50 be conduits for oxygenated groundwater to the 55 middle of the deposit. In the peat adjacent to the60 se layers, open-system conditions with respect to uranium may have been present. Occasionally samples from the central part of a deposit without visible stratigraphic peculiarities yield outlier 230 Th/U ages. A possible explanation is that permafrost might have expanded the frozen fen peat and on warming voids were created into which groundwater containing uranium could have penetrated before the peat was recompacted. Detailed descriptions of the sampling site may help correlate outlier 230Th/U ages to stratigraphic peculiarities. There are other scenarios which may occur in permafrost regions (SCHIRRMEISTER et al. 2002). For example, superficial peat layers may become dry down to a depth of 1-2 m during the summer months, allowing oxygenated melt water to enter the deep parts of the deposit. Many dated interglacial peat deposits in western Asia have been exposed along river banks. If the river rises above the level of the peat layers,

Abb. 4: Schematisiertes interglaziales Niedermoor238U profil:234U/238U Geeignete Proben für die55 230Th/U-Altersex bestimmung können nur im zentralen Bereich des 60 Torfes erwartet werden. Die Randbereiche von 10 cm Mächtigkeit im Hangenden Fig. 5und Liegenden des Torfes bilden gewöhnlich offene Systeme für Uran und erweisen sich deshalb für die 230Th/U-Datierung als ungeeignet. 50

samples collected from up to several decimetres depth may have been affected by river water. Exposed fen peat deposits well above the present river level might also have been affected by seeping rainwater. Only drill cores of sufficient length seem to guarantee suitable peat samples. Thorium typically exists in the +4 and +5 oxidation states. The ions are readily hydrolyzed and either precipitate or are adsorbed on detrital particulates (inorganic or organic), clay minerals and iron (oxy)hydroxides. Thorium ions can be transported with humic acids (RICHARDS & DORALE 2003). But movement of humic acids in confined peat beds can be excluded.

MEBUS A. GEYH

Detrital contamination with allochthonous method for this material. Accurate 230Th/U 230 Th: Airborne dust or water-borne fine-grai- ages of sublayers cannot be expected. During growth of the moss the peat remains very ned material may become incorporated during the � � � � � � � � �� and � � � � � groundwater �� or lake water contaithe formation of fen peat. These contaminants porous usually contain thorium, which is often bound ning dissolved uranium may enter even deeper to detrital clay. The presence of 230Th in the layers. Hence the 230Th/U ages will always be detrital material means that the 230Th/U clock younger than the actual ones. Moreover, as fen � � � � �� shows 230Th/U � � � ages � � � � that are too old. The allo- peat formation is restricted to shallow lakes, 230 chthonous (detrital) Th decays during ageing which exist mainly at the end of the intergla� � � � � � � � � periods, � � � or interstadial the corresponding while the content of radiogenic 230Th increases� � � �cial 234 as a result of the decay of U. Hence the organic deposits often represent the end of the 230 Th/U age error due to detrital contamination peat-formation period or the interglacial periHence, suitable decreases with increasing� age. � � � � � the � � � � beginning. �� The age error od rather � than caused by an initial detrital 230Th content is material of palynologically distinct sub-layers often large and may amount to several 1000 of the first part of the interglacial period can � � � �� be � provided. years up to 100,000 years. Therefore, the two� � � �seldom � 230 or more sources of Th (one radiogenic and at least one detrital) have to be distinguished and Exclusion of raised-bog peat from 230Th/U � � the � � 230 � � Th/U � � � ages. dating: Raised-bog peat receives uranium quantified for correction� of mainly from particulate matter in precipitation. Limits on the 230Th/U method for the dating of Therefore, the concentration is usually low and � � � � � � may � � � � � � be � � � different sources of thorium and peat and lignite: The above-described scenario � there � �� of both accumulation and binding of uranium uranium with different isotopic compositions. in fen peat limits the use of the 230Th/U dating Moreover, as raised bog peat is often only Fig. 4

238

0,4

0,5

0,6

U (dph/g) &

0,7

0,8

234

0,9

238

1,0

U AR 1,1

1,2

1,3

ash content %

1,4

25 Netiesos L/L

Netiesos L/L 30

depth (cm)

40 45 50 55

40 45 50

238U 234U/238U

Fig. 5: Depth profiles of the specific 238U activity, the 234U/238U AR and the ash content at the Netiesos site in Fig. 5 Lithuania (GAIGALAS et al. 2005). Samples L-4 and L-5 (crossed dots) have a slightly elevated specific 238U 230 activity. This indicates that the premises of Th/U dating are potentially violated. Abb. 5: Die Tiefenprofile der spezifischen 238U-Aktivität, des 234U/238U-Aktivitätsverhältnisses und des Aschegehalts vom Profil Netiesos in Litauen (GAIGALAS et al. 2005). Die Proben L-4 und L-5 (angekreuzte Punkte) haben eine leicht erhöhte 238U-Konzentration und damit ein geringes Potential, für die 230Th/UDatierung geeignet zu sein.

Th/U dating of interglacial and interstadial fen peat and lignite:Potential and limits

230

weakly decomposed and the content of humic acids is low the ability to absorb actinides is limited. As a consequence, all attempts to date raised-bog peat using the 230Th/U dating method have failed. 4 Determination of the reliability of data sets for the 230Th/U dating of fen peat and lignite Based on the above-described scenario for fen peat formation and the numerous factors that interfere with 230Th/U dating of fen peat, it is necessary to identify and to discard data sets from samples that are not suitable for 230Th/U dating. Such sample material does not fulfil the two basic premises of this dating method: (1) It has behaved as an open system with respect to uranium (and thorium) and/or (2) it contained more than one detrital contaminant. Three simple tests are used for the initial reliability determination: depth profiles of (i) uranium concentration or specific 238U activity, (ii) 234U/238U AR, and (iii) ash content. Uranium-depth profiles may show up samples which behaved as an open system with respect to uranium on the basis of elevated uranium concentration relative to the base level in the central part of the peat profile. The 234U/ 238 U AR in the top and bottom rim layers can be expected to differ from the central part if 234U was preferentially leached or accumulated. The third test is determination of the ash content of the sample: an elevated ash content may indicate the presence of sand layers. Figure 5 shows the results of the three tests for the Netiesos site (GAIGALAS et al. 2005). The 238 U activity of samples L-4 and L-5 (45 and 50 cm) are 30-40 % above the other samples and therefore these samples potentially violate the two basic premises for 230Th/U dating. The authors do not describe any disturbance of the stratigraphy, and therefore other information is necessary to determine which samples need to be excluded from the age determination. For example, the decrease in ash content with depth points to the possibility of open-system conditions in the upper part of the peat.

Examination of 222 data sets obtained from 33 fen peat sections in Eurasia (e.g. ARSLANOV 2005) has yielded evidence that 230Th/U ages can be satisfactorily correlated with the SPECMAC chronology even from samples with considerably elevated or depleted uranium concentrations, or from samples with anomalous 234U/238U ARs or that contain a high ash content. Hence, the three tests do not reliably identify samples that do not fulfil the basic premises for 230Th/U dating. More sophisticated tests are required. The theoretical background was developed in the 1960s. KAUFMAN & BROECKER (1965) introduced the concept of a binary mixture of radiogenic 230Th and allochthonous detrital 232Th. If the latter has only one source, the proportion of detrital 230Th can be estimated via the thorium concentration or the 232Th activity. As the half-life of 232Th is very large (τ = 14.01×109 a), it behaves similar to a stable isotope. Hence, its activity has not changed during the aging of the sample and hence is a measure of the initial detrital 230Th content in the sample. One of the main tasks of 230Th/U dating of fen peat and lignite is to determine the initial 230Th/232Th AR (thorium index) in order to correct the corresponding ages for the detrital contamination (Chapter 5). The principal feature of binary mixing is that any two properties of the two components show a linear correlation. Hence, the plot of any two isotope ratios of uranium and/or thorium in peat samples – according to the KAUFMAN & BROECKER concept – must also yield a straight line. If this requirement is not fulfilled, the basic premises for 230Th/U dating are not fulfilled: The material did not behave as a closed system with respect to uranium and thorium and/or there were more than two sources of detrital 230Th. In 1970, OSMOND et al. (1970) developed two mixing plots: 230Th/238U AR versus 232Th/ 238 U AR (Osmond-I plot) and 234U/238U AR versus 232Th/238U AR (Osmond-II plot) (Figs. 6 and 7 bottom). ROSHOLT (1976) introduced two additional plots, the Rosholt-I (230Th/232Th AR versus 238Th/232Th AR) and Rosholt-II plots

Th AR

2,0 Netiesos L/L

234

230

1,8

2,8

1,6

2,6

1,5

y = 1,2746x - 0,3848 R 2 = 0,876

2,4

1,4

2,2

Rosholt I

Rosholt II

2,0

1,3 1,8

2,0

2,2

2,4

2,6

2,8 238

232

1,8

3,0

2,0

2,2

2,4

2,6

2,8 238

Th AR

232

3,0 Th AR

1,3

U AR

0,80

Netiesos L/L

238

Netiesos L/L

238

U AR

3,2

y = 0,643x + 0,0263 R 2 = 0,8973

0,75

234

Th/

Netiesos L/L

3,4

3,0

1,7

230

3,6

232

1,9

Th/

232

Th AR

MEBUS A. GEYH

y = -0,289x + 1,2371 R 2 = 0,0687

1,2 0,70

0,65 1,1

y = 0,0633x + 0,6285 R2 = 0,015

0,60

Osmond II

Osmond I 0,55 0,30

1,0 0,35

0,40

0,45

0,50 232

0,55 238

Th/

0,30

0,35

0,40

0,45

0,50 232

U AR

0,55 238

Th/

U AR

Fig. 6

230

234

Th/

232

Th AR

Fig. 6: Top: Rosholt I and Rosholt II plots (ROSHOLT 1976) of the L/L data from the Netiesos site in Lithuania 1,7 (GAIGALAS et al. 2005): The slopes of the mixing lines equal the 230Th/234U AR and the 234U/238U AR, respec2,1 Netiesos TSD 230 TSD 232 234 232 tively, and the y-intercepts give the Th/ Th AR and U/ Th ARs. TheNetiesos data for sample L-6 (open circle) 230 2,0 does not fit the “isochron” and was not used for the Th/U age calculation. 1,6 1,9

Bottom: Osmond I und Osmond II plots (OSMOND et al. 1970) of the L/L data from the Netiesos site in Lithua1,8 nia (G1,5 AIGALAS et al. 2005): The slopes of the mixing lines give the 230Th/232Th AR and the 234U/232Th AR, 1,7 and 234U/238U AR. The data for sample L-6 (open respectively, and the y-intercepts provide the 230Th/238U AR y = 0,6356x + 0,351 y = 0,9794x + 0,0641 = 0,9888 circle)1,4 does not fit the mixing lines andRwas not used for1,6 the 230Th/U age calculation. R = 0,9896 2

U AR

1,5 Abb. 6:1,3Oben: Rosholt-I und Rosholt-II-Diagramme (ROSHOLT 1976) der L/L-Daten vom Netiesos-Profil in Litauen (GAIGALAS et al. 2005): Die Steigungen der beiden 1,4 Mischgeraden liefern die Aktivitätsverhältnisse Rosholt I Rosholt II 230 Th/234U und 234U/238U sowie die Y-Achsenabschnitte die 230Th/232Th- und 234U/232Th-Aktivitätsverhältnisse. 1,2 Der Datensatz der Probe L-6 (weißer Kreis) liegt jenseits1,3der „Isochrone“ und wurde bei der Berechnung des 1,3 1,4 1,5 1,6 1,7 1,8 1,9 2,0 2,1 1,3 1,4 1,5 1,6 1,7 1,8 1,9 2,0 2,1 230 238 232 238 232 Th/U-Alters nicht berücksichtigt. U/ Th AR U/ Th AR 0,94

1,07

230

234

Th/

238

U/238 U AR

Unten: Osmond-I und Osmond-II-Diagramme (OSMOND et al. 1970) der L/L-Daten Netiesos TSD vom Netiesos-Profil in Netiesos TSD 1,06 0,92(GAIGALAS et al. 2005): Die Steigungen der beiden Mischgeraden liefern die Aktivitätsverhältnisse Litauen 1,05 230 Th/232 Th und 234U/232Th sowie die Y-Achsenabschnitte die 230Th/238U- und 234U/238U-Aktivitätsverhältnisse. 0,90 1,04 Der Datensatz der Probe L-6 (weißer Kreis) liegt jenseits der Mischgerade und wurde nicht bei der Berech1,03 230 0,88 nung des Th/U-Alters berücksichtigt. 0,86

1,02

y = 0,3736x + 0,6219 R2 = 0,9221

1,01

0,84

1,00

0,82

0,99

0,55 0,30

0,35

0,40

0,45

0,50 232

1,0

0,55 238

Th/

0,30

0,35

0,40

0,45

0,50 232

U AR

Th AR

Netiesos TSD

232 234

1,6

230

Th/

232

Th AR

Fig. 6

1,7

1,9

1,7

y = 0,6356x + 0,351 R2 = 0,9888

1,4

y = 0,9794x + 0,0641 R 2 = 0,9896

1,6 1,5

1,3

1,4

Rosholt I

Rosholt II

1,3

1,2 1,5

1,6

1,7

1,8

1,9

238

1,3

2,0 2,1 232 U/ Th AR

U/238 U AR

0,92

1,4

Netiesos TSD

234

U AR

0,94

238

1,3

Th/

Netiesos TSD

2,0

1,8

1,5

230

2,1

U AR

Th/U dating of interglacial and interstadial fen peat and lignite:Potential and limits

230

0,55 238

Th/

0,90

1,07

1,5

1,6

1,7

1,8

1,9

238

2,0 2,1 232 U/ Th AR

Netiesos TSD

1,06 1,05 1,04 1,03

0,88

1,02

0,86

y = 0,3736x + 0,6219

1,01

R = 0,9221

0,84

1,00

0,82

0,99

y = 0,032x + 0,9989 R2 = 0,0414

0,98

0,80 0,78 0,45

1,4

0,97

Osmond I

0,50

0,55

0,60

0,65

0,70 232

238

Th/

0,75

0,96 0,45

Osmond II

0,50

0,55

0,60

0,65

0,70 232

U AR

238

Th/

0,75 U AR

Fig. 7 Fig. 7: Top: Rosholt I and Rosholt II plots (ROSHOLT 1976) of the TSD data from the Netiesos site in Lithuania ( GAIGALAS et al. 2005): The slopes of the mixing lines equal the 230Th/234U AR and the 234U/238U AR, respectively, and the y-intercepts give the 230Th/232Th AR and 234U/232Th AR. All data points are on the mixing lines within their uncertainty intervals and are used to calculate 230Th/U ages. Bottom: Osmond I und Osmond II plots (OSMOND et al. 1970) of the TSD data from the Netiesos data in Lithuania (GAIGALAS et al. 2005): The slopes of the mixing lines give the 230Th/232UTh AR and the 234U/ 232 Th AR, respectively, and the y-intercepts provide the 230Th/238U AR and 234U/238U AR. All data points are on the mixing lines within their uncertainty intervals and are used to calculate 230Th/U ages. Abb. 7: Oben: Rosholt-I und Rosholt-II-Diagramme (ROSHOLT 1976) der TSD-Daten vom Netiesos-Profil in Litauen (GAIGALAS et al. 2005): Die Steigungen der beiden Mischgeraden liefern die Aktivitätsverhältnisse von 230Th/234U und 234U/238U sowie die Y-Achsenabschnitte die 230Th/232Th- und 234U/232Th-Aktivitätsverhältnisse. Alle Datensätze genügen den Mischgeraden und den Kriterien der 230Th/U-Methode. Unten: Osmond-I und Osmond-II-Diagramme (OSMOND et al. 1970) der TSD-Daten vom Netiesos-Profil in Litauen (GAIGALAS et al. 2005): Die Steigungen der beiden Mischgeraden liefern die Aktivitätsverhältnisse 230 Th/232Th und 234U/232Th sowie die Y-Achsenabschnitte die 230Th/238U- und 234U/238U-Aktivitätsverhältnisse.

amplifies even a dubious isochron trend. As a result the correla with the concentration of uranium. Therefore, the scatter of the 238U/232Th AR greatly isochrons are usually higher than 0.90 and also considerably gre amplifies even a dubious isochron trend. As a result the correlation coefficient of the Rosholt plots. (2) The ARs, especially if measured radiometrically, are h 86 EBUS A. GEYH isochrons are usually higherM than 0.90 and also considerably greater than those of the Osmond denominator isotope 232Th activity is low. Due to the large analy plots. (2) The ARs, especially if measured radiometrically, are highly correlated if the lines subparallelproblem; to most isochrons paradoxically stre (234U/232Th AR versus 238U/232Th 232 AR; Figs. 6 along (“zero”-correlation CHAYESand 1949, denominator isotope Th activity is low. Due to the large analytical error the data scatter and 7 top). The Rosholt-I plot is often incorrect- of 1971). an isochron. 3) The data points of radiometrically measured ly referred to along as an lines “isochron” plot.toThe slope To obtain a well defined mixingthe line, (LUDWIG subparallel most isochrons and paradoxically strengthen visual impression to fall more precisely on the regression line than the erro of the least-squares fitted straight line in a plot of appear & T ITTERINGTON 1994), data sets from at least of an isochron. 3) The data points of radiometrically measured ARs plotted with error ranges the 230Th/232Th versus 238U/232Th activity ratios of is two coeval samples differing detrimuch more scatter with than widely the errors would otherwise indicate ( fall230 more precisely the regression line thanmust the error ranges imply, even if there coeval samplesappear equalstothe Th*/234 U AR ofonthe tal 230Th content be available. In the case CHAYES 1949, 1971). * radiogenic 230Th and its intercept thethey-axis of fenotherwise peat, a much larger number of samples is much more scatter on than errors would indicate ( zero -correlation problem; obtain well defined mixing line, (LUDWIG & TITTERINGTO equals the decay-corrected initial 230Th/232Th AR To should beaanalysed. CHAYES 1949, (detrital correction factor = 1971). f). The Rosholt- two Coeval samples with contamicoeval samples withsimilar widelydetrital differing detrital 230Th content 234 238 II plot yields To theobtain present U AR. Both yield data cluster,1994), whichdata excludes the at least a wellU/ defined mixing line, nation (LUDWIG & TaITTERINGTON sets from fen peat,of a much larger number of samples should be analyse ARs are required to calculate the 230Th/U ages of possibility a mixing line. Moreover, the y-axis 230 two coeval samples with widely differing detrital Th content must be available. In the case (Eq. 1). An “isochron” slope of 0 corresponds Coeval intercept of the with mixing line must becontamination positive, as yield a data samples similar detrital of of fenzero; peat, an a much largerage number should be negative analysed.initial 230Th/232Th AR to a 230Th/U age “infinite” has a of samples a decay-corrected possibility of a mixing line. Moreover, the y-axis intercept of th slope of 1 (radioactive equilibrium). is theoreticallyyield excluded. The mean of negative Coeval samples with similar detrital contamination a data cluster, which excludes 230 the as a decay-corrected negative initialonly Th/232Th AR is Figures 6 and 7 show two sets of the four positive, decay-corrected AR values can be accepted possibility of a mixing line. Moreover, the y-axis intercept of the mixing line must be Rosholt-Osmond plots of the ARs obtained mean if its confidence includes AR zero.values can be accepted o of negativeinterval decay-corrected from the uranium and as thorium analyses ofnegative the The four230mixing plot tests – especiallyexcluded. those by The positive, a decay-corrected initial Th/232Th AR is theoretically zero. fen peat at the Netiesos site using the L/L and includes Osmond – are a sensitive check for identifying mean of negative decay-corrected AR values can be accepted only if its confidence interval TSD methods. The displacement of any pair The unsuitable data plot sets.tests However, it isthose recomfour mixing especially by Osmond are a includes of AR values from anyzero. of the four mixing lines mended to include the three simple tests deidentifying unsuitable data sets. However, it is recommended to indicates that The the four corresponding may scribed above in a rigorous examination mixing plot sample tests especially those by Osmond are a sensitive check of forthe in aage rigorous examinationsuppleof the reliability of U not fulfill the basic premises for 230Th/U dat- described reliabilityabove of U/Th data. Moreover, identifying unsuitable data sets. However, it is recommended to include the three simple tests ing. Especially, the Rosholt I and II plots give supplementary mentary tests should be found whichwhich also allow tests should be found also allow an objec “a misleadingdescribed impression of inana well-defined an objective and reproducible identification of above rigorous examination of the reliability of U/Th age data. Moreover, of suitable data and unsuitable data sets. isochron for data of little statistical power” identification suitable and unsuitable sets. supplementary tests should be found which also allow an objective and reproducible (LUDWIG 2003). LUDWIG mentions three factors ofTh suitable and unsuitable data5sets. as the reason. identification (1) The 238U/232 and 234 U/232 Th Detrital contamination and the 5 Detrital contamination and the correction factors ARs tend to reflect the U/Th element ratio. correction factors The abundances of both 234U and 230Th of Detrital contamination and theAccording correctiontofactors non-zero-age 5materials are generally highly theKKAUFMAN AUFMAN & BROECKER (1965) According to the & BROECKER (1965) concept, the r 230 correlated with the concentration of uranium. concept, the radiogenic Th activity ([230Th]*) 230 can be calculated the measured activity [230Th] Therefore, the scatter of the 238U/232Th AR ([ can Th]*) be calculated from thefrom measured activity 230 According to the KAUFMAN & BROECKER (1965) concept, the radiogenic Th activity greatly amplifies even a dubious isochron [230Th] as follows: ([230Th]*) can be calculated from the measured230 activity [230 Th] as 232 follows: � � � t 230 trend. As a result the correlation coefficient * 230 ( 2 ) [ Th ] � [ Th ] � [ Th] � f o � e 230 � [ Th] � f � [ 23 of the Rosholt isochrons are usually higher than 0.90 and also considerably greater than (2) [ 230Th(2) ]* �The [ 230Th ] � [ 232 Th] � f o � e � �230 � t � [ 230Th] � f � [ 232Th] , those of the Osmond plots. ARs, especially if measured radiometrically, are highly correlated if the denominator isotope 232Th where fo is the initial 230Th/232Th AR and f is the activity is low. Due to the large analytical er- decay-corrected fo. ror the data scatter along lines subparallel to Any correction of radiometrically determined most isochrons and paradoxically strengthen 230Th/U ages for detrital 230Th is negligible if the visual impression of an isochron. 3) The the measured 230Th/232Th AR of any sample is data points of radiometrically measured ARs smaller than 20 because in this case the detrital plotted with error ranges appear to fall more 230Th activity is very low. precisely on the regression line than the error The detritus-corrected 234U activity ([234U]*) ranges imply, even if there is much more scat- can be calculated from the Rosholt II plot using ter than the errors would otherwise indicate an analogous equation:

230 an analogous Th/Uequation: dating of interglacial and interstadial fen peat and lignite:Potential and limits

[ 234U ]* [ 234U ] [ 232Th] g o e O234 t 234 t

[ 234U ] g [ 232Th] ,

where go is initial 234U/232Th AR and g is the decay-corrected go. Both of these equations to correct for detrital thorium and detrital uranium assume there was only source of contamination. No detrital correction is needed if most of the samples contain little or no detrital contamination and inferring effects can be excluded, like in the case for speleothems and or corals. In such cases the slope of the mixing lines of the Rosholt-I and Rosholt-II plots equal the 230Th/238U AR and 234U/238U AR, respectively, which are required to calculate the 230Th/U age (Equation 1). The computer program ISOPLOT (LUDWIG 2003) can be used to calculate the corrected 230 Th/U age and its standard deviation. The theoretical background for calculating the “error-correlated” standard deviation is given by LUDWIG & TITTERINGTON (1994) and LUDWIG (2001; 2003).

U AR

1,25

1,1 1,2

Netosos-LL

238

1,30

80 kyr corrected

1,15 Netiesos TSD 1,10

1,20 U AR

exc

1,05

1,15

238

In the case of fen-peat dating a statistical check of the individual 230Th/U ages is recommended in order to identify outliers. This cannot be done with the ISOPLOT program. Therefore, Equations 2 and 3 should be applied to each data set of peat and lignite samples (GEYH 1994; 2001). The required f and g values and their standard deviations are obtained from the intercept of the mixing lines with the y-axis of the Rosholt-I and Rosholt-II plots or the iterative approximation of the decay-corrected initial 230 Th/232Th AR described below. The standard deviation of the detritus-corrected 230Th activity is calculated using the usual error propagation equations. In most cases the correction for detrital uranium is not necessary because the 234 U/238U AR and 232Th/238U AR do not correlate for most peat samples. This is also the case for the Netiesos site (Figs. 6 and 7 bottom). An independent approach to the determination of the detrital correction factor f is to approximate the decay-corrected initial 230Th/232Th AR iteratively. The 230Th/232Th AR and 234U/232Th AR

234

(3)

234

1,00 1,1

1,10

1,05

0,95

80 kyr

1,05

1,00 0,45

corrected

0,50

0,55

0,60 230

Th/

0,65 238

U AR

0,70

0,75

0,80

0,90 0,45

0,55

0,65

0,75

0,85 230

0,95 238

Th/

U AR

Fig. 8: Isotope-ratio evolution plot: Scatter of theFig. L/L8 (left) and TSD ARs and their standard deviations in 230 232 234 232 the Th/ Th- U/ Th diagram for the Netiesos site in Lithuania (GAIGALAS et al. 2005). This scatter decreases with increasing detrital correction factor. The L/L data of sample L-6 (crossed symbols) remains far from the cluster. Abb. 8: Isotopen-Evolutionsdiagramm: Die Streuweite der L/L- und TSD-Aktivitätsverhältnisse des Netiesos-Proﬁls in Litauen (GAIGALAS et al. 2005) nimmt im 230Th/232Th- 234U/232Th-Diagramm mit steigendem detritischen Korrekturfaktor ab. Die L/L-Daten der Probe L-6 (gekreuzter Punkt) bleibt der Punkthäufung fern.

MEBUS A. GEYH Fig. 8

3,83850

Netesios L/L (5133-5137) 3,83845

(230Th/U) (ka)

2,4

Netiesos TSD

2,2 2,0 1,8

3,83840

1,6 3,83835

3,83830 0,414

1,4

0,415

0,416

0,417

0,418

0,419

0,420 0,421 0,422 thorium index

1,2 0,40

0,45

0,50

0,55

0,60

0,65

0,70

thorium index

Fig. 9 of the iteratively corrected L/L and TSD 230Th/U Fig. 9: The scatter represented by the standard deviations ages of the Netiesos site in Lithuania (GAIGALAS et al. 2005) approaches a minimum at f(L/L) = 0.412  0.080 and f(TSD) = 0.534  0.011. The values for sample L-6 were discarded.

Abb. 9: Streuweite der iterativ-korrigierten 230Th/U-Alter vom Netiesos-Proﬁl in Litauen (GAIGALAS et al. 2005) erreicht Minima bei f(L/L) = 0.412  0.080 bzw. f(TDS) = 0.534  0.011. Der Datensatz der Probe L-6 wurde ausgeschlossen.

of coeval samples containing contamination from different sources scatter widely. Iteratively increasing the detrital correction factor f causes the points to move to the left in the isotope-ratio evolution plot of KAUFMAN & BROECKER (1965). The higher the detrital contamination the faster the points move to the left. The optimum detrital correction factor f is obtained when the points of the cluster are the closest together. The points in the cluster move further apart when the f value is increased beyond the value for the minimum width of the cluster. An elliptical shape of the cluster is caused by differences in the initial 234 U/238U AR (Fig. 8). An effect resulting from -recoil as observed in carbonates (HENDERSON & SLOWEY 2000) cannot be completely excluded for fen peat. A plot of the standard deviation of the 230Th/U age versus the f value provides a measure of the scatter of the individual ages. For the case study from the Netiesos site , the scatter of the L/L and TSD 230Th/U ages approaches a minimum at f(L/L) = 0.412  0.080 and f(TSD) = 0.534  0.011 (Fig. 9). The plots again show that sample L-6 does not fulﬁll the premises for 230Th/U dating. An iterative method developed to determine the decay-corrected initial 230Th/232Th AR provides an additional test for a rigorous examination of the reliability of the data sets (GEYH 2001). The iteratively detritus-corrected 230Th/U ages

plotted against the iteratively determined detrital correction factor yields lines with different slopes. If all samples behaved as closed systems with respect to uranium and only one detritus component was present in the samples, the lines should intersect at only one point – the actual f value. If one or more lines do not pass through this point the corresponding sample is identiﬁed as unsuitable. For the case study of the Netiesos site, the bold line for sample L-6 in the L/L plot belongs to outlier data which do not fulﬁll the premises of the 230Th/U dating method. This line does not pass through the common intersection point. 6 230Th/U age and random uncertainty The data sets suitable for calculation of a 230Th/U age and its standard deviation are selected on the basis of the rigorous examination of their reliability described above. The detrital correction is applied to each measured 230Th activity (Equation 2; Chapter 5) and the 230Th/U ages are calculated using Equation 1. The C2 test is then applied to the corrected 230Th/U ages, outliers are discarded, and the mean 230Th/U age is calculated. The standard deviation of the mean 230 Th/U age is obtained using the Gaussian method for error determination of the 230Th/U ages of the samples.

Th/U dating of interglacial and interstadial fen peat and lignite:Potential and limits

230

Fig. 9 Th/U age (ka)

Netiesos L/L

5133

5134

5135

5137

230

Th/U age (ka)

100

5136

5138

120 Netiesos TSD

110

5129 5130 5131

100

5132

30 20

10 0 0,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1,0

1,1 1,2 1,3 1,4 thorium index

60 0,30

0,35

0,40

0,45

0,50

0,55

0,60

0,65

0,70

thorium index

Fig. 10: Change of the detritus-corrected 230Th/UFig. ages10with the iteratively increased f. Five lines of the L/L data and all lines of the TSD data have common intersection points at 61.6 ka and 76.4 ka, respectively. The bold line of the sample L-6 does not. Abb. 10: Ă&#x201E;nderung der detritus-korrigierten 230Th/U-Alter bei iterativ erhĂśhtem f-Wert. FĂźnf Linien der L/LDaten und alle der TSD-Daten haben nahe zusammen liegende Schnittpunkte bei 61.6 ka und 76.4 ka. Die dicke Linie des L/L-Datensatzes der Probe L-6 liefert davon entfernte Schnittpunkte.

7 Minimum 230Th/U age of samples The limited precision of uranium and thorium isotope activity measurements means that there is an upper limit of the 230Th/U age that can be determined. This limit is the minimum 230 Th/U age of a sample, corresponding to the maximum 230Th/U age that can be determined with the 230Th/U dating method. This minimum 230 Th/U age is expressed, for example, as >350 ka. Surprisingly, minimum ages are usually not estimated. The isotope-ratio evolution plot of KAUFMAN & BROECKER (1965) can be used to develop a method for calculating the minimum 230Th/U age (GEYH & MĂ&#x153;LLER 2005). X in Equation 1 is replaced by 230Th/238U AR and Y by the decaycorrected initial 234U/238U AR. Thus,

(4)

O230 (Y 1) Â&#x2DC; (1 O23o O234

1) Â&#x2DC; (1 e ( O230 O234 ) Â&#x2DC;t ) (1 e O23 o Â&#x2DC;t ) The decay-corrected initial given by

(5)

234

(Yo 1) Â&#x2DC; e O234 Â&#x2DC;t 1

U/238U (Y) is

The uranium and thorium ARs of an inďŹ nitely old sample are given by a straight line in the isotope-ratio evolution plot. The equation for this isochron is derived from Equation 4 assuming an age t = â&#x2C6;&#x17E;: (6)

O230 O234 Â&#x2DC;Y 1 O230 O234 O230 O234

The ellipse deďŹ ned by the X and Y random uncertainties of any data point should touch but never cut the inďŹ nite-age isochron. When the inďŹ nite-age isochron is tangent to the ellipse of the X and Y random uncertainties, the minimum 230Th/U age tmin of the sample can be determined from the Xmin and Ymin (Fig. 11). The equation of the ellipse with maximum axis lengths of Âą 2SX and Âą 2SY around Xmin and Ymin is given by 2

(7)

Â§ X X min Âˇ Â§ Y Ymin Âˇ Â¨Â¨ Â¸Â¸ Â¨Â¨ Â¸Â¸ ÂŠ 2VX Âš ÂŠ 2VY Âš

Equations 4-7 are solved iteratively by decreasing or increasing the age until the error ellipse of a date touches the inďŹ nite-age isochron. A detritus-corrected 230Th/U date is considered reasonable if it is smaller than the corresponding minimum 230Th/U sample age tmin (GEYH &

MÜLLER 2005). Otherwise the result has to be expressed as the minimum 230Th/U age.

234

8 Precision and accuracy of 230Th/U dating of fen peat

U/ 238 U activity ratio

MEBUS A. GEYH

The precision and accuracy of 230Th/U dating of fen peat may be illustrated by three case studies. GAIGALAS et al. (2005) published L/L and TSD 230Th/U ages of 108.8 ± 8.7 ka and 80.3 ± 5.9 ka, respectively, for fen peat deposits at the Netiesos site in Lithuania. The higher value from the L/L data was obtained after rejection of the data from the uppermost (L-1: 30 cm) and lowermost (L-6: 55 cm) peat samples. A rigorous examination of the sample data sets was not carried out. The decision to reject these two values was based neither on an elevated 238 U activity nor on ash content (Fig. 5). A rigorous check of the reliability of the L/L data sets identified only sample L-6 as unsuitable for 230 Th/U dating. The L-6 data do not fit the mixing lines of the Rosholt-I and Osmond-I plots (Fig. 6), no minimum cluster size was found when the L-6 data was included in the evolution plot (Fig. 8), and in the iterative plot the L-6 line does not pass through the common intersection point of the L-1 to L-5 lines. Sample L-2 has a slightly elevated ash content (Fig. 5) and its AR values do not fit well the mixing lines of the Rosholt-I and Osmond-I plots (Fig. 7). Other indications that L-2 is unsuitable were not found. A rigorous check of the reliability of the TSD data sets does not identify any outliers. The 230 Th/U age was calculated with the Equations 1 and 2 and yielded 78.4 ± 4.3 ka and a detrital correction factor of 0.519 ± 0.161. The χ2 test applied to the four data sets yields χ2 = 0.4. Iterative evaluation yielded an age of 76.4 ± 1.3 ka with a detrital correction factor of 0.534 ± 0.010. A 230Th/U age of 72.6 ± 10.0 ka and a detrital correction factor of 0.268 ± 0.365 and a χ2 = 0.8 were calculated using Equations 1 and 2, discarding the data from samples L-2 and L-6. Iterative evaluation yielded a detrital correction factor f = 0.456 ± 0.060. An age of 77.0 ± 11.1 ka and a detrital correction factor f = 0.199 ± 0.381 (χ2 = 0.4) were obtained when only the

1,3

sample point 1,2

(Xmax ,Ymax )

1,1

isochron of infinite age

isotopic evolution line

1,0 1,0

1,1

1,2 230

Th/ 238U activity ratio

Fig. 11: Isotope-ratio evolution plot (KAUFMAN & Fig. 11 BROECKER 1965) to determine the minimum 230Th/U age tmin of a sample. When the data point (open circle) moves on the evolution line to the right with increasing age (grey circle) until its ellipse of 2σ X and Y uncertainties touches the infinite-age isochron (grey circle), the corresponding Xmin = 230Th/238U AR and Ymin = 234U/238U AR yield the minimum age tmin. Abb. 11: Isotopenentwicklungsdiagramm nach KAUFMAN & BROECKER (1965) zur Bestimmung des methodischen Minimum-230Th/U-Alters einer Probe. Der weiße Punkt bewegt sich mit wachsendem Alter auf der Entwicklungslinie bis zu dem grauen Punkt, an dem die Ellipse der 2σ Y- und Y-Standardabweichungen die Isochrone für unendliche Alter trifft (schwarzer Punkt). Die entsprechenden Xmin = 230Th/ 238 U- und Ymin = 234U/238U- Aktivitätsverhältnisse liefern das Minimum-230Th/U-Alter der Probe.

data from sample L-6 was discarded and the iterative evaluation yielded f = 0.419 ± 0.080. Disregarding sample L-6 shifts the calculated lines in the Rosholt-I and Osmond-I of the L/L data plots to the right and downwards, respectively. This yields an acceptable 230Th/U age of 77.0 ± 11.1 ka. It is in agreement with the TSD 230 Th/U age of 78.4 ± 4.3 ka. This age falls within the time span of MIS 5a (Brørup interstadial) between 76 and 78 ka of the SPECMAP time scale (MARTINSON et al. 1987).

Th/U dating of interglacial and interstadial fen peat and lignite:Potential and limits

230

The second case study presented here was the first attempt to check whether it is possible to date fen peat and lignite by the 230Th/U dating method (GEYH 2001). Eemian peat deposits in Europe seemed to be the most suitable for this as they contain the only Quaternary interglacial material that can be palynologically firmly identified and related to the SPECMAP timescale (GEYH 2001). Unfortunately, no cores were available and all samples were collected from exposures with the associated risk of interfering processes. Seven sites in Germany yielded a mean 230Th/U age of 111.9 ± 1.2 ka for the detritus-corrected 230 Th/U ages and 114.4 ± 1.9 ka using Equations 1 and 2. The agreement is good but the ages are below what is expected from SPECMAP chronology. There are several possible reasons for this. The data sets were not subjected to a rigorous reliability test. The samples might not have completely fulfilled the two basic premises for 230Th/U dating due to the lack of optimum sampling conditions. In any case, the 230Th/U ages and their standard deviations for these seven sites may be considered as representative of the attainable accuracy of 230Th/U dating of fen peat. The precision, however, is good enough to firmly assign the dated Eemian interglacial deposits to MIS 5e but is not sufficient to assign a precise chronological age to sub-layers. In the third case study, the 230Th/U ages for Holsteinian/Hoxnian Interglacial peat were obtained from drill cores and agree better with the SPECMAP chronology (GEYH & MÜLLER 2005). The Hoxnian Interglacial in England has been correlated with the Holsteinian Interglacial (TURNER 1970). A mean 230Th/U age of 317 ± 14 ka was obtained from the fen peat at Tottenhill in the Nar Valley of NW Norfolk (ROWE et al. 1997). The 230Th/U ages from the Marks Tey site in the UK, which has been palynologically classified as Holsteinian, (ROWE et al. 1999), have a very low precision and could be interpreted only by using a probabilistic approach. The authors concluded that these deposits correlate with MIS 11 or some older stage with 87 % confidence. Using Equations 4 to 7, 230Th/U minimum ages of >165 to >245 ka

were obtained for their data (GEYH & MÜLLER 2005). This implies that the measured U and Th data reliably allow only the statement that the geological age of the Holsteinian samples is greater than MIS 7. B The Bossel site about 30 km west of Hamburg contains two organic layers: one above and one below the sediments of the Holsteinian Sea. This location became the reference site for the Holsteinian Interglacial in northern Germany as declared by the European Commission on the Stratigraphy of the Quaternary. The “isochron” plot of the AR of ten samples from both peat layers yielded a mean detritus-corrected 230 Th/U age of 323 ± 5 ka with individual detritus-corrected 230Th/U ages between 298 and 347 ka. The lower peat layer yielded a mean detritus-corrected 230Th/U age of 312 ± 3 ka with individual detritus-corrected 230Th/U ages between 293 and 369 ka. The four 230Th/U ages of the upper peat layer yielded a detritus-corrected mean age of 327+50/-37 ka. These case studies provide evidence that 230Th/ U dating of fen peat and lignite does not yield high-precision ages. The accuracy may be around 10,000 a. But this is adequate to reliably relate any interglacial peat deposit to only one of the documented warm periods of the SPECMAP chronology (MARTINSON et al. 1987). 9 Sampling, analysis and isotope ratio determination To fulfil the requirements for isochron230Th/U dating the samples must be coeval from the same interglacial or interstadial. For this it is sufficient to take samples from anywhere within the centre of the fen peat deposit. Rim layers of at least 10 cm thickness have to be excluded from the analysis. It is recommended to take the samples from a monolith or a drill core. At least four samples with a wide range of concentration of the detrital contamination, but even better more should be dated. The sample preparation is more or less the same for both measurement methods for determining the ARs and consists of several steps. Ultrapure

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acids which are free of even traces of uranium and thorium have to be used. The chemical preparation should be done in a clean-air laboratory in order to lower the risk of any detrital contamination. This is especially important if small samples are treated for measurement in a TIMS (thermal-ion mass spectrometer) or MCICP-MS (multi-collector inductively coupled plasma mass spectrometer). In radiometric laboratories repeated and comprehensive contamination tests have to be carried out. Preferentially, peat should be treated by “total sample dissolution” (TSD) (BISCHOFF & FITZPATRICK 1991; LUO & KU 1991). This method includes both lattice-bound and adsorbed thorium. Empirically it has been found that 230Th/U ages obtained by the “leachate/leachate” method (SCHWARCZ & LATHAM 1989; KAUFMAN 1993) often deviate from the palynologically expected age and do not fit the SPECMAP timescale. Selective leaching of peat samples does not adequately separate the radiogenic and detrital 230Th components. In addition, the radiogenic 230Th may become readsorbed on detrital material during incomplete dissolution. The following steps are taken in the “total sample dissolution” method: − The samples of 3-5 g of fresh or 0.3-0.5 g of dry peat are taken from visibly undisturbed micro ranges within monoliths or cores. − The surface of each sample is removed. The peat is then broken into pieces, dried and ignited in a quartz tube at a maximum temperature of about 800 °C in a stream of oxygen to burn all organic material. Insoluble glass beads do not form below this temperature and, therefore, loss of uranium and thorium is avoided. − The weighed peat ash is treated with NaOH in order to remove traces of humic acids. The residue is completely dissolved in a con. HF/HNO3/HCL mixture to completely dissolve uranium and thorium. If the L/L technique is applied, the sample is leached with a HNO3/HCl mixture for at least 6 h. Next, 0.5 mL of a 229Th spike and 0.5 mL of a 233U/236U double spike are added as a check of thermal fractionation in the ion source.

− − − −

The solution is heated with an infrared lamp for up to 20 h in order to ensure perfect mixing of sample and spike. The leachate is separated from the residue by centrifuging. Uranium and thorium are co-precipitated with Fe(OH)3. The precipitate is dissolved in con. HNO3. Uranium and thorium are separated from the mixture by standard ion-exchange chemistry using an actinide-specific resin (DOWEX 1×8 100-200 mesh). Thorium is eluted from the column by dilute HCl, uranium is then removed with HBr.

Th/U age determinations are preferentially done now by TIMS and MC-ICP-MS. Details are given by SCHOLZ & HOFFMANN (2008). Radiometric measurements of the ARs require at least ten times more material and are less precise by one order of magnitude. The extracted uranium and thorium are electroplated on stainless steel discs. The latter are put into an alpha spectrometer and the emitted alpha particles are counted. Due to the long 230Th half-life of 75,690 ka, only one out of 6×106 atoms of 230Th decays during the common measurement time of one week. Hence, especially the counting statistics limits the precision of radiometric 230Th/U ages. 230

10 Conclusions Interglacial fen peat and lignite with ages of up to 350-500 ka can be dated with the 230Th/U dating method. The main prerequisite is that the samples are collected from the parts of a deposit that behaved as closed system with respect to uranium during ageing and have no more than one detrital component. In the case of fen peat, the bottom and top rim layers of about 10 cm have to be discarded. Drill cores yield better samples than monoliths collected from exposed outcrops. Small samples of a few cm3 (about 1 g of dry peat) are sufficient for TIMS and MC-ICP-MS 230Th/U dating. Radiometric 230 Th/U dating requires at least ten times more material. The coeval samples should have a wide range in the amount of detrital contamination. A

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single uranium and thorium isotope analysis of peat does not yield a reliable 230Th/U age. The TSD analytical method seems to be superior to the L/L method. Reliable 230Th/U ages, however, always require a comprehensive and rigorous reliability check of the measured data in order to identify samples that were unsuitable for 230Th/U dating. The accuracy of 230Th/U ages of fen peat and lignite is around 10 ka, which is sufficient for reliable correlation with the warm periods of the global marine δ18O chronology. Acknowledgement I sincerely thank Dr. Denis Scholz, University Heidelberg. He read the paper very critically and made many valuable suggestions which substantially improved the manuscript. Clark Newcomb kindly overtook the final reading. References ARSLANOV, KH. A. (2005): Interim Report INTAS Project 01-0675 of the CR2 Team. May 1, 2004 to April 30, 2005 (unpublished). BISCHOFF, J.L. & FITZPATRICK, J.A. (1991): U-series dating of impure carbonates: An isochron technique using total-sample dissolution. – Geochimica Cosmochimica Acta, 55: 543-554. BOURDON, B., HENDERSON, G.M., LUNDSTROM, C.C. & TURNER, S.P. (eds.) (2000): Uranium-series Geochemistry. – Reviews in Mineralogy and Geochemistry, 52: 656 pp.; Washington, D.C. (Mineralogical Society of America). CHAYES, F. (1949): On ratio correlation in petrography. – Journal of Geology, 57: 239-254. CHAYES, F. (1971): Ratio Correlation. A Manual for Students of Petrology and Geochemistry: 99 pp.; Chicago (University Chicago Press). CHENG, H., EDWARDS, R.L., HOFF, J., GALLUP, C.D., RICHARDS, D.A. & ASMERON, Y. (2000): The halflives of uranium-234 and thorium-230. – Chemical Geology (Isotope Geoscience Section), 169: 17-33. GAIGALAS, A., ARSLANOV, K.A., MAKSIMOV, F.E., KUZNETSOV, V.Y., CHERNOV, S.B. & MELESYTÉ, M. (2005): Results of uranium-thorium isochron dating of Netiesos section peat-bog in south Lithunia. – Geologija, 51: 29-38. GEYH, M.A. (1994): Precise “Isochron”-Derived Detritus-Corrected U/Th Dates. – 16th Radiocarbon

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